1. Field of the Invention
The present invention relates to an automatic transmission for transmitting rotation of an input shaft to an output shaft at a speed selected from a plurality of shift speeds by making each element of first and second compound planetary gears capable of being connected to the input shaft through a clutch, and also capable of being fixed through a brake.
2. Description of the Related Art
Japanese Patent Application Publication No. JPA-2002-161951 (pp. 5 to 8, FIG. 5) discloses an automatic transmission that includes a double-pinion type simple planetary gear unit that has a carrier supporting a pair of intermeshing pinions, a sun gear meshing with one of the pair of pinions, and a ring gear meshing with the other pinion, and also includes, on the common axis line, a compound planetary gear unit that has a former-stage and a later-stage sun gears, a former-stage and a later-stage ring gears, a former-stage carrier supporting a pinion meshing with the former-stage sun gear and the former-stage ring gear, and a later-stage carrier supporting a pinion meshing with the later-stage sun gear and the later-stage ring gear, and that directly interconnects the former-stage and later-stage sun gears and also directly connects the former-stage carrier and the later-stage ring gear. The automatic transmission directly connects an output shaft to the later-stage carrier, selectively transmits rotation of an input shaft to the directly connected former-stage and later-stage sun gears through a first clutch, selectively transmits the rotation of the input shaft to the directly interconnected former-stage carrier and later-stage ring gear through a second clutch, and selectively transmits the rotation of the input shaft to the sun gear of the simple planetary gear unit through a third clutch. The automatic transmission also includes a first brake directly connecting the ring gear of the simple planetary gear unit to the former-stage ring gear of the compound planetary gear unit to fix the carrier of the simple planetary gear unit, a second brake fixing the directly interconnected ring gear of the simple planetary gear unit and former-stage ring gear of the compound planetary gear unit, the third brake fixing the sun gear of the simple planetary gear unit through a one-way clutch, and a fourth brake fixing the directly interconnected former-stage carrier and later-stage ring gear of the compound planetary gear unit. Thus, the automatic transmission establishes five forward speeds and one reverse speed gear ratios by selectively engaging the first, second, and third clutches, and the first, second, third, and fourth brakes.
Japanese Patent Application Publication No. JPA-2002-213545 (p. 5, FIGS. 1 and 2) discloses an automatic transmission in which a common sun gear directly connected to an input shaft is meshed with a first ring gear through a small-diameter pinion of a stepped pinion supported by a carrier, and in which a compound planetary gear for deceleration meshed with a second ring gear through a large-diameter pinion of the stepped pinion is directly connected with a sun gear of a first single-pinion planetary gear and with a sun gear of a second single-pinion planetary gear. The automatic transmission is provided with a compound planetary gear for shifting in which a carrier of the first single-pinion planetary gear and a ring gear of the second single-pinion planetary gear are directly interconnected, a first clutch selectively connecting the input shaft with a directly connected sun gear of the compound planetary gear for shifting, a second clutch selectively connecting the input shaft with the directly connected carrier and ring gear of the compound planetary gear for shifting, a first brake selectively fixing a first ring gear of the compound planetary gear for deceleration, a second brake selectively fixing a second ring gear of the compound planetary gear for deceleration, a third brake selectively fixing a directly connected carrier of the compound planetary gear for deceleration and a ring gear of the first single-pinion planetary gear, a fourth brake selectively fixing the directly connected carrier and ring gear of the compound planetary gear for shifting, and an output shaft directly connected to a carrier of the second single-pinion planetary gear. Thus, the automatic transmission changes rotation of the input shaft into one of eight forward speeds and one reverse speed, and transmits the rotation to the output shaft.
The automatic transmission described in JP-A-2002-161951 can shift between five forward speeds and one reverse speed gear ratios in a smooth and stable manner. However, in recent years, there are demands for multi-speed automatic transmissions to improve fuel consumption and power transmission performance, or to obtain gear ratios suited to the driver's taste. The automatic transmission described in JP-A-2002-213545 responds to such demands by enabling to obtain gear ratios of eight forward speeds and one reverse speed.
However, referring to the gear ratios of the automatic transmission disclosed in JP-A-2002-213545, as shown in its FIGS. 1 to 3, the step ratio between shift speeds (gear ratio on the lower speed side/gear ratio on the higher speed side) is 1.638 between the first forward speed and the second forward speed, 1.243 between the second forward speed and the third forward speed, 1.085 between the third forward speed and the fourth forward speed, 1.509 between the fourth forward speed and the fifth forward speed, 1.529 between the fifth forward speed and the sixth forward speed, 1.053 between the sixth forward speed and the seventh forward speed, and 1.115 between the seventh forward speed and the eighth forward speed. Particularly, the step ratios between the third forward speed and the fourth forward speed, and between the sixth forward speed and the seventh forward speed are as small as less than 1.1. Thus, the step ratio of less than 1.1 indicates that the gear ratio changes only slightly between before and after shifting, and especially in driving, a driver might not be able to get a shift feeling (feeling that a shift is performed) in spite of shifting, thus, being given an uncomfortable feeling.